1. Field of the Invention
The present invention is generally directed to an optical disc and an apparatus for driving the optical disc, and more particularly, both to an improvement of a preformat system which is marked beforehand for servo on the optical disc and to an optical disc driving apparatus suited to this improved preformat system.
2. Description of the Prior Art
Turning to FIG. 1, there is illustrated a diagram of a track sector format of a conventional optical disc which is shown, for instance, in "Optical Mass Data Storage 2", appearing on p. 112, Vol. 695 (1986) of the journal "SPIE". It can be observed from FIG. 1 that each track 200 per cycle is composed of 32 sectors (#0 through #31). A sector consists of 43 blocks (B1 through B43). Each individual block is constituted by a 2-byte servo field followed by a 16-byte data field. Hence, one track is split into 1376 blocks which is given by: 32.times.43=1376. FIG. 2 shows pit patterns of the servo field depicted in FIG. 1. Pits 201 and 203 are positioned above and below the under the center axis of a track 206, and pits 202 and 203 are above and below the center axis of another track 207. Pits 204 and 205 are clock pits. Tracking sensor signals can be obtained only from these pairs of wobbled pits. This type of servo system is referred to as a sampled servo, the principle of which is described, e.g., in "Third International Conference on Optical Mass Data Storage" appearing on p. 140, Vol. 529 (1985) of the journal "SPIE". A more detailed description is therefore omitted here. In an optical disc based on such a conventional system, it is feasible to obtain the tracking sensor signals only from pairs of pits within the servo fields, thereby requiring that no guide groove be designed for tracking. When performing accessing from one track to another at a high velocity, different servo field structures A and B, as described in FIG. 2, are alternately disposed for every 16 tracks. As a result, the amount of track-movement during accessing can be counted. In FIG. 2, the track number is given by: EQU I+(N-1).times.16
where I=1, 2, 3, ... 16. In the servo field structure A, N=1, 3, 5 ..., and in the servo field structure B, N=2, 4, 6, ... In the servo field structures A and B, the pits 201 and 202 of the two pairs of pits deviate in the track direction. In the case of effecting accessing while obliquely traversing the track, the number of tracks traversed can be obtained by detecting the positions of the pits. This situation will be explained by referring to FIG. 3. In FIG. 3, a multiplicity of central track lines 208 depicted in lateral solid lines are arranged at a spacing of, for example, 1.5 .mu.m. In connection with the servo fields indicated by vertical dotted lines 209, the structures A and B are, as shown at the right end in the figure, alternately arranged for every 16 tracks. When providing a high-speed access, and assuming that a light spot travels along a locus 210, the light spot comes to intersect the servo fields at the points 211. The servo field structure can therefore be recognized from these points 211. Shown is one example of the thus recognized signal waveform 212. A "high" level indicates the servo field structure A, while a "low" level indicates the servo field structure B. Each time a state changes in signal waveform 212, it follows that 16 tracks have been counted. It is possible to count the number of tracks crossed by the light spot on the basis of the signal waveform 212 during the accessing, enabling an optical head to immediately reach the target track.
On the other hand, as is obvious from FIG. 3, a defect inherent to the sampled servo system is caused in that it is not feasible to detect whether the light spot is travelling toward the outer periphery or the inner periphery of the optical disc. The high-speed access involves the use of a method of controlling the velocity by fetching a speed detecting signal during the accessing of the optical head. This speed controlling method has more advantages than the well-known method of controlling the velocity with a glass scale provided outside. More specifically, the advantages provided are that such a glass scale is not needed, that miniaturization can be attained, and that a moderation in the required mechanical precision is permitted. Where this speed controlling method is applied to the prior art optical disc, however, a critical defect is caused in that directional detection is not possible. The reason for this will be elucidated. If an access direction is inverted during speed control, this directional inversion cannot be detected. Consequently, a control loop is put into a positive feedback state, resulting in runaway of the optical head. Such being the case, there is a possibility that the optical head will impinge upon a stopper provided on the inner or outer peripheral portion of the optical disc and will thereby be broken. Since the above-described conventional technique employs the servo field structures which vary alternately for every 16 tracks, assuming that the rotational frequency of the disc is 1800 rpm, the number of tracks can be counted up to this high velocity given by: 16.times.track pitch (1.5 .mu.m)/block cycle (1/30.times.1/1376 sec)=1.0 m/sec. On the other hand, it is impossible to count the number of tracks if it is less than 16. For this reason, if the number of remaining tracks approximates to 16, there is no alternative but to employ one of the low speed track count techniques, this leading to a great obstacle to the desired reduction in access time. The low speed track count technique herein implies a method of counting the number of times the tracks are crossed on the basis of the tracking sensor signal of the sampled servo, where the maximum limit detection velocity is given by: track pitch/block cycle=61.9 mm/sec. When controlling the velocity by fetching a speed detecting signal from the optical disc during the accessing of the optical head, the detection of the speed signal is permitted when the optical head has travelled 16 tracks. Hence, the dead time of the speed detector increases, thereby making the speed control system unstable. In addition, broad band high speed control becomes impractical.
The sampled servo system utilizes a servo clock for the purpose of forming a trigger pulse signal to sample the first and second wobbled pits, a byte clock for sectioning the byte unit for data demodulation and a main clock serving as the reference from a PLL circuit synchronizing with the clock pits. In order to detect the clock pits, the distance (19-bit length) between the second wobbled pit 203 and the clock pits 204 and 205 is set longer than the maximum distance (18-bit length) between pits in the data pattern. Thus, the pit detected immediately after the distance between two adjacent pits has exceeded the given value is identified as the clock pit, thus enabling detection of the clock pits. Differential processing of reproduced signals transmitted from the optical disc is a general practice for detecting clock pits. If data information pits consecutively exist on the inner portion of the optical disc as continuous bits, however, the optical frequency characteristics are poor and in some cases the reproduced peaks are not separated. In this case, the continuous pits are detected as an independent pit, and the distance between pits seemingly increases. Hence, if the apparent distance between pits in the data pattern exceeds 19 bits, it follows that an error in detection of the clock pits is created, and that the PLL does not normally function. Accordingly, the sampling of the sampled servo is not favourably effected, and the tracking servo does not work well. As a result, the optical head runs in a disordered fashion towards the inner and outer peripheries of the disc, and the record/reproduction operation cannot be effected properly. Furthermore, a problem arises in that if the clock pits are mistakenly detected during recording, and the synchronization of the PLL fails, false recording will be carried out.